1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a method of receiving downlink data in a wireless communication system, and a wireless device using the method.
2. Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) evolved from a universal mobile telecommunications system (UMTS) is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) evolved from the 3GPP LTE.
As disclosed in 3GPP TS 36.211 V8.7.0 (2009-05) “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)”, a physical channel of the LTE may be classified into a downlink channel, i.e., a physical downlink shared channel (PDSCH) and a physical downlink control channel (PDCCH), and an uplink channel, i.e., a physical uplink shared channel (PUSCH) and a physical uplink control channel (PUCCH).
To decrease an interference caused by uplink transmission between user equipments (UEs), it is important for a base station (BS) to maintain uplink time alignment of the UEs. The UE may be located in any area in a cell. An uplink signal transmitted by the UE may arrive to the BS at a different time according to the location of the UE. A signal arrival time of a UE located in a cell edge is longer than a signal arrival time of a UE located in a cell center. On the contrary, the signal arrival time of the UE located in the cell center is shorter than the signal arrival time of the UE located in the cell edge.
To decrease the interference between the UEs, the BS needs to perform scheduling so that uplink signals transmitted by the UEs in the cell can be received every time within a boundary. The BS has to properly adjust transmission timing of each UE according to a situation of each UE. Such an adjustment is called an uplink time alignment. A random access process is one of processes for maintaining the uplink time alignment. The UE acquires a time alignment value (or also referred to as a timing advance (TA) through the random access process, and maintains the uplink time alignment by applying the time alignment value.
In addition, a transmit power of the UE needs to be adjusted to mitigate an interference caused by uplink transmission. It is difficult for the BS to receive uplink data if the transmit power of the UE is too low. If the transmit power of the UE is too high, uplink transmission may cause a significant interference to transmission of another UE.
Recently, multiple serving cells are introduced to provide a higher data rate. However, the same time alignment value has been applied to all serving cells under the assumption that serving cells have adjacent frequencies or have similar propagation properties.
A reception timing difference among a plurality of serving cells may be increased greatly when configuring the plurality of serving cells to which different time alignment values are applied.